The present invention is directed to process to prepare sulfonate polyimides by melt reaction of an amino sulfonic acid or amino sulfonic acid salt with at least one molten polyimide resin. The process provides sulfonic acid or sulfonate salt functionalized polyimides with improved flow.
Polyimide resins with sulfonate salt functionality incorporated into the repeat units or end groups of the polymer chain, polyimide ionomer resins, have been prepared, for example in U.S. Pat. No. 4,588,805, and are of current interest in fuel cell membrane and other related systems. Polyimides with bonded sulfonic acid functionality are also useful materials for instance as exchange membranes and may be converted into various sulfonate salts by neutralization. Incorporation of sulfonate functionality, either sulfonic acid or sulfonate salt, bonded to the polymer can substantially modify polymer properties such as gas permeability, solubility and moisture absorption.
Sulfonate polyimides are usually made by multistep methods combining various monomers to build the polymer from smaller units usually in several steps using sulfonated aromatic dianhydride and/or sulfonate aryl diamine monomers as part of the polymerization process. Alternate methods involve sulfonation of preformed polyimides in solution U.S. Pat. No. 4,574,144. These are multistep processes often employing expensive and or potentially hazardous materials requiring solution polymerization. Such processes require relatively long reaction times. Processes using solvent further require that the sulfonated polymer be separated from solvent and the polymer dried of solvent. The solvent must them be recovered and purified for reuse or treated as waste. Often polyimide polymerizations require expensive aprotic polar solvents such as N-methyl pyrrolidone, dimethyl formamide, dimethyl acetamide or halogenated aromatic compounds such as chlorobenzene or dichloro benzene. Additionally starting from monomer units, polymerization to build a high molecular weight resin must be accomplished without interference from or destruction of the sulfonate functionality.
Polyimide resins are difficult to melt process, the amorphous polyimide resins have a high glass transition temperature (Tg), often above 200° C., and polyimides with useful mechanical properties are high molecular weight species. Such polyimides are generally are more difficult to melt process and form into finished articles than higher flowing crystalline resins or lower Tg amorphous thermoplastics.